1. Field of the Invention
This invention pertains to a method and apparatus for processing a semiconductor wafer and, more particularly, to a method and apparatus for identifying and controlling the impact of ambient conditions on photolithography processes.
2. Description of the Related Art
Semiconductor devices, or microchips, are manufactured from wafers of a substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the electrical circuits that make up the device. The fabrication essentially comprises four basic operations. The four operations are:
layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced;
patterning, or removing selected portions of added layers;
doping, or placing specific amounts of dopants in the wafer surface through openings in the added layers; and
heat treatment, or heating and cooling the materials to produce desired effects in the processed wafer.
Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process. Indeed, whole texts have been written on the many ways in which these operations can be employed. See, e.g., Peter Van Zant, Microchip Fabrication A Practical Guide to Semiconductor Processing (3d Ed. 1997 McGraw-Hill Companies, Inc.) (ISBN 0-07-067250-4).
Patterning is also sometimes called photolithography, photomasking, masking, oxide removal, metal removal, and microlithography. The term xe2x80x9cphotolithographyxe2x80x9d will hereafter be used to refer to patterning operations. In photolithography, typically, a machine called a xe2x80x9cstepperxe2x80x9d positions a portion of a wafer being processed under a xe2x80x9creticle,xe2x80x9d or photomask. A reticle is a copy of a pattern created in a layer of chrome on a glass plate. Light is then shone onto the wafer through the reticle. The chrome blocks some of the light. The light shining through the pattern on the reticle changes the material characteristics of the wafer where it contacts the wafer. These changes make the material more or less susceptible to removal in another operation, depending on the particular process being implemented. The stepper then positions another portion of the wafer under the reticle, and the operation is repeated. This process is repeated until the entire wafer has undergone the operation.
The purpose of photolithography is to create in or on a wafer the parts of what will ultimately be the semiconductor device. The parts must be laid down in the exact dimensions, within certain manufacturing tolerances, required by the circuit design and to locate them in their proper place. Some people consequently consider photolithography to be the most critical of these four basic operations.
The photolithography operations generally set the xe2x80x9ccritical dimensionsxe2x80x9d of the semiconductor devices. Errors can cause a whole host of problems including, but not limited to, distorted patterns, misplaced patterns, and other defects. These types of errors can undesirably change the functioning of the electrical circuits so that the wafer has to be scrapped. Photolithography processes are performed at very small dimensions, so that they are also highly susceptible to contamination by particulates or unwanted variations in processing conditions.
One type of processing condition is called an xe2x80x9cambientxe2x80x9d condition. Ambient conditions in the processing facility, such as air pressure, temperature, and humidity, significantly impact the performance of semiconductor photolithography tool. Variations in the ambient conditions may deleteriously affect the operation of the tool because photolithography techniques need to account for the effect such conditions have on the operation, especially when they vary. However, there is no known mechanism for controlling photolithographic processes to account for this impact.
The present invention is directed to resolving one or all of the problems mentioned above.
The invention is, in its various aspects, a method and apparatus for processing a semiconductor wafer. More particularly, the invention is a method and apparatus for identifying and controlling the impact of ambient conditions on photolithography processes. In a first aspect, the invention is a method for processing a semiconductor wafer. The method comprises identifying a disturbance in a photolithographic process arising from an ambient condition; modeling the identified disturbance; and applying the model to modify a control input parameter. In a second aspect, the invention is an apparatus for controlling a photolithography process. The apparatus includes an exposure tool, including a photolithography controller; and a computer receiving data from the exposure tool. The computer is programmed to perform a method comprising: identifying a disturbance in a photolithographic process arising from an ambient condition; modeling the identified disturbance; and applying the model to modify a control input parameter.